This invention relates to magnetrons.
Referring to FIG. 1 of the accompanying drawings, which is an axial section through a known magnetron, a known magnetron consists of a hollow anode 1 into which a cathode indicated generally by the reference numeral 2 extends. RF power may be coupled out of the anode into a waveguide (not shown) by coupler A housed in ceramic dome B. Input power is provided by a HT d.c. power supply 3 between the cathode and the anode, with the anode typically being at ground potential and the cathode at a high negative potential. The interaction space between the anode and cathode is evacuated and, in order to hold off the HT voltage between the anode and cathode, a sleeve 4 of insulating material forms part of the vacuum envelope. The sleeve 4 is bonded to the anode and cathode, respectively, by alloy sleeves 5, 6. The cathode is hollow, and consists of an outer sleeve 7 containing a core 8, and the emissive part of the cathode is a bright emitter helical filament 9. To complete the vacuum envelope at its upper end, an outwardly-flared region 10 of the cathode sleeve is bonded to the end of the core 8 by means of alloy sleeves 11, 12, which are separated from each other by an insulating sleeve 13. The sleeves 11, 12 are made of Kovar, a nickel cobalt ferrous alloy, in order to have a coefficient of thermal expansion compatible with that of the insulating sleeve 13, which is of ceramic material. A power supply to heat the filament is applied between the head of the core and the flared portion of the cathode outer sleeve. The power supply includes an isolation transformer indicated generally by the reference numeral 14, the primary of which is driven by the mains C, and also earthed, the output of the secondary being superimposed on the high negative voltage applied to the cathode by d.c. supply 3.
The transformer operates at mains frequency, but this is a disadvantage, because the insulation between primary and secondary is heavy and bulky.
It would be preferred to operate transformer 14 at high frequency, because the size and weight of the transformer would be greatly reduced.
However, this would have the disadvantage of causing significant heating and power loss because power will be dissipated in the material of the alloy sleeves 11, 12.
Thus, a high frequency supply from the secondary of the transformer 14 would generate a high frequency alternating current travelling along the core 8 and returning along the flared region 10. Since Kovar is a ferromagnetic material, significant magnetic flux would be generated circulating through the bulk of the sleeve 12, also alternating at high frequency. This in turn would generate currents in the sleeve 12, which would cause power loss. The same situation applies to sleeve 11.
It has been proposed in JP3187129 to provide a capacitor type HV input terminal to a magnetron, which input terminal is coated with a conductive layer and carries a high frequency filament current.